Developments have conventionally been made on non-aqueous electrolyte secondary batteries, or generally known as lithium ion batteries, that employ a transition metal oxide such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide or lithium iron phosphate as a positive electrode active material and a layered carbon compound such as artificial graphite or natural graphite as a negative electrode active material.
Generally, secondary batteries tend to exhibit favorable characteristics in the initial period of use and such characteristics gradually deteriorate owing to the repetition of charge and discharge. Accordingly, non-aqueous electrolyte secondary batteries are required to maintain the same level of characteristics as in the initial period of use over a long period of time and to improve their reliability. Cycle characteristics, load characteristics etc. can be used as an index of the battery reliability. These characteristics are largely affected by side reactions that occur in the interface between the positive electrode and the electrolyte and the interface between the negative electrode and the electrolyte, and by diffusibility of the ions in the non-aqueous electrolyte.
In order to improve the cycle characteristics and load characteristics of a non-aqueous electrolyte secondary battery, for example, the use of a fluorine-substituted ether as a non-aqueous solvent for non-aqueous electrolyte has been proposed. The fluorine-substituted ether is a compound in which all or some of the hydrogen atoms of the ether are substituted by fluorine atoms. By substituting fluorine atoms for hydrogen atoms, the viscosity is reduced, and the electrochemical oxidation resistance is improved. For this reason, fluorine-substituted ethers are considered usable as a non-aqueous solvent for a high energy density secondary battery having an output voltage of around 4 V.
Japanese Laid-Open Patent Publication No. Hei 11-026015 (hereinafter referred to as “Document 1”) has proposed a non-aqueous electrolyte that contains a fluorine-substituted ether (hereinafter referred to as “fluorine-substituted ether (A)”) represented by General Formula (A): RF1—CH2O—RF2, where RF1 is a linear fluoroalkyl group having at least one fluorine atom and 2, 4, 6 or 8 carbon atoms, and RF2 is a linear fluoroalkyl group having at least one fluorine atom and 2 or 3 carbon atoms. The fluorine-substituted ether (A) is compatible with a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC) or the like, so by being mixed with such a cyclic carbonate when used, the cycle characteristics and low temperature discharge characteristics of the non-aqueous electrolyte secondary battery can be improved.
However, owing to its low electrochemical reduction resistance, the fluorine-substituted ether (A) is likely to cause a side reaction with a negative electrode active material and take lithium, which is usable in battery reactions, from the negative electrode active material. For this reason, the inclusion of the fluorine-substituted ether (A) in a non-aqueous electrolyte can cause the cycle efficiency of the negative electrode to drop. Consequently, a battery with a sufficient cycle characteristics cannot be obtained.
U.S. Patent Application Publication No. 2007/0054186 (hereinafter referred to as “Document 2”) has proposed an electrolyte composition that contains a solvent composition that contains a fluorine-substituted ether (hereinafter referred to as “fluorine-substituted ether (B)”) having an oxy (methylmethylene) group [—CH(CH3)O—] in a molecule thereof and an electrolyte salt. As a specific example of the fluorine-substituted ether (B), a compound that has a divalent group represented by a formula: —CF2—CH(CH3)—O— in a molecule thereof is described. Document 2 also discloses that the fluorine-substituted ether (B) has a superior electrochemical oxidation resistance, that even when a battery using the above electrolyte composition is retained at a high voltage, the internal resistance does not increase and is maintained at a low level, and that the use of the above electrolyte composition can improve the cycle characteristics and low temperature load characteristics of the battery.
In addition, Document 2 demonstrates carbonates like EC, PC, diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) as solvents that can be used with the fluorine-substituted ether (B) in the above solvent composition, and describes that the fluorine-substituted ether (B) content in the above solvent composition is, for example, 20 to 90% (see [0071]).
From the viewpoint of avoiding the deterioration of cycle characteristics, load characteristics and the like of the battery, a single-phase non-aqueous electrolyte is required, but the fluorine-substituted ether (B) essentially has poor solubility in carbonates. Thus, in order to obtain a single-phase solvent composition, Document 2 discusses combinations of the fluorine-substituted ether (B) with carbonates, and discloses, as specific examples of such combinations, a combination of the fluorine-substituted ether (B) with EMC or DEC, and a combination of the fluorine-substituted ether (B) with EC and EMC or DEC in the examples thereof.